There exists many forms of data storage devices for use with digital computers. It has now become appropriate, for some applications which require particularly intensive use of storage capability, to design mass storage devices which can maintain and store indefinitely large quantities of data. One form in which such a mass storage device can be implemented is an optical disk system. Commercially available write-once, read-many (WORM) systems are available. For example a two sided 30 centimeter diameter glass optical disk of the type developed by the Philips Corporation can be read and write on a Lazer Magnetic Storage International 1200 Read/Write Unit, with each WORM optical disk having a storage capability of 1 gigabyte (2.sup.30) bytes of data.
The overall storage capacity of an optical reader based mass storage device can be improved by using many optical disks with a small number of units for reading and writing on the disks. Unfortunately, the use of such a strategy means that the information on most of the disks is, at all times, off-line. This complicates the user interface to the disks and also makes more complicated functions which a user may normally expect, such as access to disk directories and searches of various file attributes.